RF and microwave semiconductor chips (transistors, DUT's) are best characterized “on wafer”, before cutting, bonding and assembling them at high cost. This also allows avoidance of parasitic connection elements, like wire bonds and fringe capacitors, which are associated with packaging the devices in order to mount them in test fixtures. It also allows a much larger number of devices to be tested “in situ” without having to laboriously slice the wafer, mount and wire-bond each individual chip. The “on wafer” testing is at this point of time the preferred testing method, except for high power devices, beyond 10-20 Watt RF power. On-wafer testing is also the exclusive testing method in millimeter-wave frequencies, since device packaging is extremely difficult and the parasitic elements associated with the package (inductance of wire bonds and fringe capacitors of package housings) could falsify the measured data to the point of uselessness.
A number of manufacturers (see ref. 1, 2) make wafer probes capable of reliably testing chips in GHz range frequencies. The wafer probes (FIG. 1, item (1) are made, usually, using small coaxial or coplanar sections (10) with diameters of the order of 1 mm (0.04″) that end into a “coplanar” structure (4), (shown also in detail in FIG. 7b), where the center conductor of the coaxial section (cable) becomes the center conductor of the coplanar section and the ground mantle of the coaxial section (cable) ends up as the ground plane of the coplanar (FIG. 7b). The simple reason is that this is a practical way for the RF signal to be injected and retrieved from planar micro-chips, where all RF contacts are on the same surface (Ground-Signal-Ground or GSG configuration). Back in FIG. 1, it is visible how the coaxial cable is coupled into a coaxial connector (8) and attached to a coaxial cable (2) which leads to the test instrument.
Therefore at least three connection points (probe tips) are necessary to establish a proper RF contact in GSG (Ground-Signal-Ground) configuration (FIG. 6b). Since three probe tips do not necessarily form a straight line and since the probes themselves may be, microscopically, imperfect, due to the tiny dimensions involved (the typical gap between the three probe tips of the order of 150 micrometers ˜0.006″) and because of unavoidable manufacturing tolerances, it often happens that the contact between the probe tips (21, 70, 71, 72), and chip contact plots (23, 24) is disoriented (FIG. 2a) and uneven and unreliable (73, FIG. 7a). Prior art probe holder (FIG. 21) does not provide for a lateral disorientation correction (“Phi” angle), defined in FIGS. 2 and 6. Up to now such disorientation was corrected by rotating the whole probe holder (FIG. 21) on its magnetic base. No fine adjustment has been possible. The present disclosure does not claim inventorship on the whole apparatus of FIG. 21. The apparatus of FIG. 21, up to the left limit of the dotted contour (211) is considered here “fixed support” (FIG. 1),